Field of the Disclosure
The present disclosure relates to an extensometer with a passive vertical system making use of a linear optical encoder for use in materials testing.
Description of the Prior Art
An extensometer is an instrument that accurately measures the dimensional changes of a test specimen under an applied load in order to better capture the material properties of the specimen. The most common type of extensometer measures axial strain, meaning the change in length of the specimen as it is stretched under a load. This is done by tracking two points along the length of a specimen starting at a precise initial separation or gauge length. The percentage difference between the initial and final separation between those two points is the axial strain.
Additionally, there are transverse strain extensometers which typically work in combination with axial extensometers. Transverse extensometers track the lateral edges of the test specimen as it stretches axially. During materials testing, the specimen's cross section will get smaller under tensile load. This change in width or diameter is the transverse strain the extensometer measures. The accuracy requirement of transverse extensometers is driven by testing standards and can be as stringent as requiring one half of one micrometer, or micron, of accuracy (0.5 μm). The transverse strain is measured at the midpoint between the axial gauge length points being tracked by the axial extensometer.
As with axial extensometers, there are contacting and non-contacting types of transverse extensometers. Contacting extensometers physically track the specimen edges during the test with two arms, whereas non-contacting extensometers typically rely on imaging to track the dimensional changes in the specimen.
For automatic contacting extensometers it is typically necessary to attach and detach the contacting arms from the test specimen. It is also typically necessary to employ a measurement system sophisticated enough to provide the accuracy required.
The instrument is typically designed to achieve the necessary accuracy as a complete system.
A transverse extensometer operates in conjunction with an axial extensometer and its contacting points typically must remain in the midpoint of the two axial contact points. The challenge is that the axial points both move in the direction of the moving end of the test specimen, and the precise vertical location is unknown. Therefore, the device typically must either calculate the vertical position from data from the axial extensometer and drive it there, or attach to the specimen and let the specimen carry it along as it stretches. If this were not done, the extensometer would slip with respect to the specimen resulting in erroneous data.
It is of importance to note that a specimen must be, in effect, immune to any external load acting upon it from an extensometer to the extent that material property data resulting from the test is statistically unaffected. This requirement typically applies to any kind of extensometer. While larger specimens subjected to large loads may be essentially unaffected by an extensometer for example the mass of a lightweight manual clip-on extensometer, smaller specimens subjected to smaller loads will register this external force during the test and produce bad data.
As a result, a driven system often makes use of sophisticated mechanisms and sensitive sensors that ensure accurate vertical positions and prevent unwanted external loads. The simpler and preferred method is to use a carefully counterbalanced, low-friction, passive tracking system where the specimen motion is moving the extensometer. This can be challenging given the typical size of a transverse extensometer measurement unit.
Given the measurement accuracy and automatic requirements of the device, the measurement unit is usually located as close as possible to the specimen, otherwise greater effort must be placed on the design to ensure faithful tracking of the measurement from the specimen to the measuring system.
The accuracy requirement can often be very strict, and for this reason, highly sensitive technologies have been employed in the past. Inductive sensors, strain gauges, and magnetic scales are some of the technologies used. These technologies, however, come at the expense of limited measurement ranges, usually in the order of just a few millimeters. In many cases resolution is also not sufficient to achieve the stringent accuracy requirements. In addition, their packaging tends to create large and heavy units, which, in combination with the actuators needed for automating the necessary motions, lead the designs towards the driven architecture mentioned earlier for vertical positioning and tracking.
To fulfill the requirement for automatic motion, the extensometer must make certain distinct motions: (1) move vertically to the desired position, (2) attach/detach the contacting arms onto the specimen edges, and most likely (3) move in and out of the test area so as to not interfere with the loading and removal of the test specimen, though it theoretically could be avoided or combined with another motion.
Current automatic, contacting strain extensometers include the multiXtens transverse strain extensometer of Zwick and the MFQ-A of MF. Prior art extensometers include HRDE of the assignee of the present application. All these devices use driven vertical systems and measurement systems with limited range. FIGS. 1-4 illustrate various prior art extensometers 1000.